I am missing (or maybe I just can't found) some commands in the API like e.g. 'Create/Edit a Tag' , 'Create/Edit PermSets' or 'Edit Sorting information of groups'.
For example, I can only import a tag from a xml file, but this is not an option for me. The same applies to permission sets.
Is there an alternative way to do these things with the API?
Unfortunately there is not a direct API call for what you are asking. The only way to do it would be to export the settings you are wanting to alter, make the changes and then re-import.
Since you stated that importing a tag from an xml file was not an option, I am not sure what your better choice would be.